While working on something else, I noticed that debug_tree (vec), when
vec is a TREE_VEC was crashing because TREE_NOTHROW was asserting that
its argument is not a TREE_VEC, so print_node would crash.

It turned out that TREE_NOTHROW was accidentally modified by this
change set:

commit 87d8f7b67c6a36c37e48e298f26e693520099b1e
Author: rguenth <rguenth@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Tue Aug 21 10:03:38 2012 +0000

    2012-08-21  Richard Guenther  <rguenther@suse.de>

    	cp/
    	* cp-tree.h (TREE_INDIRECT_USING): Use TREE_LANG_FLAG_0 accessor.
    	(ATTR_IS_DEPENDENT): Likewise.
    	(ARGUMENT_PACK_INCOMPLETE_P): Use TREE_ADDRESSABLE instead of
    	TREE_LANG_FLAG_0 on TREE_VECs.

    	* tree.h (struct tree_base): Add union to make it possible to
    	re-use the upper 4 bytes for tree codes that do not need as
    	many flags as others.  Move visited and default_def_flag to
    	common bits section in exchange for saturating_flag and
    	unsigned_flag.  Add SSA name version and tree vec length
    	fields here.
    	(struct tree_vec): Remove length field here.
    	(struct tree_ssa_name): Remove version field here.

    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@190558 138bc75d-0d04-0410-961f-82ee72b054a4

Richard Guenther pre-approved the reverting of the TREE_NOTHROW change
line.

Tested on x86_64-unknown-linux-gnu against master and applied to
trunk.

gcc/
	* tree.h (TREE_NOTHROW): Use the base.nothrow_flag.

From-SVN: r190595